J . Am. Chem. SOC.1987, 109, 2833-2834 analyses. Especially noteworthy is the efficient conversion of Boc-Tyr-Ala-Thr-OBzl to its phosphonate (entry I): the threonine residue failed to interfere in either of the transformations. To demonstrate the applicability of 1 .+ 3 4 to large (hydrophilic) peptides, we carried out both reactions in hydroxylic solvents containing K H C 0 3 or K2CO3 as a base. Thus, Z-AlaTyr-OEt in 95% aqueous methanol converted to Z-Ala-Tyr(Tf)-OEt in 88% yield, and the triflate in ethanol gave Z-AlaPhe[4-PO(OEt),]-OEt in 85% yield. Phosphonation of simple aryl triflates was general and efficient (Table I, entries 6-15). It was slowed by steric hindrance (entries 8-1 1) and was fastest with electron-deficient substrates (entries 7, 12-15) and with the electron-rich estrone triflate (entry 6 ) . With m-bromophenyl triflate, C 4 bond reactivity predominated (entry 14) and we found that it was matched only by that of the aryl C-I bond. In a competition experiment, diethyl phosphite converted ca. 40% of phenyl triflate and 60% of phenyl iodide to the phosphonate. Reactant concentrations, initially equimolar in CD3CN, were estimated by I3Cand ‘H N M R before and after reaction. In summary, diethoxyphosphinyl groups replaced tyrosine hydroxyls via sequential trifluoromethanesulfonylation and Pd(0)-catalyzed coupling with diethyl phosphite. The process has wide scope, functionalized several prototypical tyrosyl peptides without detectable racemization, and was compatible with hydroxylic solvents. It is therefore an unusual development in peptide chemistry because processes capable of drastically modifying complex polypeptides to novel, nonproteinogenic derivatives are rare. Moreover, we believe it is thefirst formation of an aryl C-P bond in a peptide framework and augurs further modifications of peptides by related Pd-catalyzed processes. We are currently devising methods to convert tyrosyl polypeptides in aqueous media to the corresponding phosphonic acids and related compounds.
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Acknowledgment. We are indebted to Dr. Richard Friary for discussions and decisive assistance with this manuscript. We also thank Drs. B. Pramanik, P. R. Das, and M. Puar for spectra and Professor R. Breslow and Drs. P. P. Trotta and J. G. Berger for discussions.
Discrete Trinuclear Complexes of Niobium Related to the Local Structure in Nb3C18 F. Albert Cotton,* Michael P. Diebold, and Wieslaw J. Roth Department of Chemistry and Laboratory for Molecular Structure and Bonding, Texas A&M University College Station, Texas 77843 Received January 9, I987 The three lower halides of niobium, Nb3X8 (X = C1, Br, I) have been known for a very long time.’ Their structures consist of close-packed arrays of halide ions with the niobium ions occupying triangular sets of adjacent octahedral interstices; bonding between the neighboring niobium atoms creates triangular metal atom clusters. The structure can be viewed as one in which Nb3(p3X)(pX),X, units, with the structure a, are fused together by
a (1) (a) v. Schnering, H. G.; Wohrle, H.; Schafer, H. Natunvissenshafren 1961,48, 159. (b) Simon, A.; v. Schnering, H. G. J. Less-Common Met. 1966, 11, 31.
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C
Figure 1.
ORTEP drawing of the Nb,C1,(PMe,Ph)6 molecule. The thermal ellipsoids are drawn at 30% probability level. Carbon atoms have been assigned arbitrarily small thermal parameters for the sake of clarity.
sharing the terminal C1 atoms. A very similar structure exists in compounds of the type Z ~ , M O , O ~ .In~ 1978 it was first reported that the Mo3(p3-O)(p-0),L9 type of unit could be obtained as a discrete entity3=and the chemistry of such species has since been developed e x t e n ~ i v e l y . We ~ ~ now wish to report a similar development with respect to the haloniobium species, namely, the preparation and characterization of discrete trinuclear complexes containing the Nb3(,q-Cl)(p-CI), core surrounded by nine ligands. The compound, Nb3C1,(PMe2Ph),, was prepared4 where T H T is by reduction of [NbC12(THF)]2(p-Cl)2(p-THT), tetrahydrothiophene, with sodium amalgam in T H F in the presence of PMe2Ph. The reaction gave a red brown solution from which the brown product was isolated in 45% yield after evaporation to small volume and addition of hexane. Crystallographic a n a l y s i ~has ~ . ~been carried out on the material obtained by recrystallization from toluene/hexane. The asymmetric unit comprises a discrete trinuclear complex, shown in Figure 1, and a molecule of toluene.’ Each trimer has virtual C3 symmetry and is chiral; two molecules of each chirality are present in the unit cell. Selected interatomic dimensions averaged according to C3 symmetry are as follows: Nb-Nb, 2.831 [3] A; (2) Torardi, C. C.; McCarley, R. E. Inorg. Chem., 1985, 24, 476 and references therein. (3) (a) Bino, A.; Cotton, F. A,; Dori, Z. J. Am. Chem. SOC.1978, 100, 5252. (b) Birnbaum, A,; Cotton, F. A.; Dori, Z.; Kapon, M.; Marler, D.; Reisner, G . M.; Schwotzer, W. J . Am. Chem. SOC.1985, 107, 2405 and references therein. (4) All operations were performed under inert atmosphere by using standard vacuum line techniques. ( 5 ) Crystal structure determinations were carried out by standard methods, which have been described previously: (a) Bino, A,; Cotton, F. A,; Fanwick, P. E. Inorg. Chem. 1979, 18,3558. (b) Cotton, F. A,; Frenz, B. A.; Deganello, G.; Shaver, A. J. Organomet. Chem. 1973, 50, 227. (6) Calculations were done on the VAX-11/780 computer at the Department of Chemistry, Texas A&M University, College Station, TX, with the VAX-SDP software package. The crystallographic analysis was carried out at room temperature and Mo K n (A = 0.71073 A) radiation was used. (7) The compound crystallized in a monoclinic space group P 2 , / n with a = 12.339 (2) A, b = 27.275 (8) A, c = 19.187 (9) A, 0 = 92.76 (2)O, V = 6450 (6) A,, Z = 4, and dcalcd = 1.491 g/cm’ forfw = 1447.9. Refinement of 605 parameters using 4805 reflections with > 3 a ( P ) produced residuals R and R, equal to 0.054 and 0.070, respectively. The tables of atomic positional and thermal parameters, bond distances, and bond angles are provided as supplementary material. (8) The only comparable structures that we are aware of are those of [M~,S~Cl,(drnpe)~]+ (Cotton, F. A,; Llusar, R., in press) and the [Ir,H,(dppp),]*+ ion (Wang, H. H.; Pignolet, L. H. Inorg. Chem. 1980, 19, 1470).
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Nb-CI,, 2.468 [4] A; Nb-Clb(trans to P), 2.529 [4] A; NbClb(trans to CI), 2.474 [4] A; Nb-CI,, 2.483 [7] A; Nb-P(trans to clb), 2.695 [2] A; Nb-P(trans to CI,), 2.729 [7] A; where subscripts c, b, and t refer to capping, bridging, and terminal chlorine atoms, respectively. The metal-ligand distances show trans influences: Nb-CI, trans to P is longer than Nb-CI, trans to CI, and Nb-P trans to CI, is longer than Nb-P trans to CI,. The cluster has eight electrons in the M, core, and based upon the electronic configuration of the M3XI3molecules (vide infra), it is expected to have a singlet ground state and consequently be diamagnetic. This was confirmed by the determination of its magnetic susceptibility9 which gave a value of xMequal -(560 f 60) X cgs at room temperature. The proton N M R spectrum" of the complex was uninformative, with unresolved peaks for both phenyl and methyl protons. Proton-decoupled phosphorus-3 1 signals were observedll only at low temperatures. The spectrum recorded a t -90 OC in T H F consisted of a sharp peak at -46 ppm due to free phosphine and two broad resonances centered around -10 and -3.5 ppm due to the inequivalent sets of coordinating phosphines. While the width of the signals may to some degree result from partial substitution of the ligands by the solvent (thus giving a number of mixed THF/phosphine species), we believe that the broadening is mainly or entirely due to the quadruplar moment of ',Nb nuclei. This nuclide has been reported" to cause anomalous and unexpected effects in the N M R spectra of some compounds of Nb. Analogous V and Ta species did not exhibit such problems. In general it is not possible to predict in which compounds the interaction with N b nuclei is going to affect the N M R behavior since the phenomenon is not well understood. Clearly, the preparation and investigation of the Ta trimers would shed some light on this problem. The compound reported here differs from Nb,C18 in the number of electrons present in the Nb, cluster, but the difference is consistent with a long-established understanding of the electronic structures of compounds with this type of structure.I3 The molecular orbitals that can be assigned primarily metal atom cluster character (in contradistinction to metal-ligand or ligand character) are three: an e and an a l orbital which correspond to three M-M u bonds and another a l orbital of somewhat higher energy that is approximately nonbonding in the M-M sense. On this basis, the ideal population of the cluster would be six electrons, which would occupy the e and lower a, bonding M O s . However, one or two more more electrons can be accommodated without severely destabilizing the structure.I4 In the case of Nb3C18there are seven electrons and in the type of compound described here there are eight. The reported' dimensions of Nb3C1,, which has crystallographically imposed 3m symmetry, are Nb-Nb, 2.810 A; Nb-CI,, 2.438 A; Nb-Clb, 2.428 A; and Nb-CI,, 2.634 and 2.522 A. In this case the metal-metal and metal-capping chlorine distances are somewhat shorter than in the phosphine complex. Such small differences may arise from several sources: the fact that in Nb3C18 the CI atoms belong to a continuous array and are shared between clusters; the presence of some phosphine ligands in place of CIligands; the occurrence of different numbers (seven and eight) of cluster electrons. Preliminary results obtained with other phosphines indicate the formation of trinucler clusters analogous to the PMe2Ph complex. In particular the use of PBu3 followed by reaction of the product
with dmpe, bis(dimethylphosphino)ethane, afforded an analogous trinuclear compound with bidentate ligands chelating the N b atoms. Acknowledgment. We are grateful to the Robert A. Welch Foundation (Grant 494) for financial support. M.P.D. is a National Science Foundation Fellow and also holds a Texaco/IUCCP Fellowship from this department. Supplementary Material Available: Tables of positional and isotropic-equivalent thermal parameters, bond distances, and bond angles for Nb,Cl,(PMe,Ph)&H, (6 pages). Ordering information is given on any current masthead page.
DNA, Light, and Dewar Pyrimidinones: The Structure and Biological Significance of TpT3 John-Stephen Taylor* and Michael P. Cohrs Department of Chemistry, Washington University St. Louis, Missouri 63130 Received December 19, 1986 The cis-syn and (6-4) dipyrimidine photodimers are the major products produced by irradiation of DNA at 254 nm and are considered to be the principal photolesions leading to mutation and cancer by ultraviolet light.'V2 The existence of perhaps another biologically relevant class of lesions comes from reports in the early 1970s of type I11 photoreactivation of the lethal effects of 254-nm light on a variety of bacterial strain^.^ Type 111 photoreactivation was shown to be most efficient at 313 nm and to proceed through an unknown, though nonenzymatic, pathway which was correlated with the disappearance of (6-4) products. Interestingly,, in 1964 Johns et al. had reported that the (6-4) product of TpT, known to them only as TpT4, could be converted quantitatively a t 3 13 nm to a new photoproduct, TpT3, and back again at 240 nm (see Scheme 1): Such photochemical behavior is quite similar to that observed for simpler pyrimidinones which have been shown to be photoisomerized to their Dewar isomers at wavelengths greater than 300 Herein we report the remarkable structure of TpT3, which we have determined to contain a highly strained
"'OH
TpT3 Dewar pyrimidinone subunit and which we show can be produced by direct photolysis of TpT with biologically relevant wavelengths of light. ~
~
~~
( I ) See: ( a ) Friedberg, E. C . DNA Repair; Freeman, New York, 1985. (9) A magnetic susceptibility balance manufactured by Johnson Matthey (b) Wang, S. Y., Ed. Photochemistry and Photobiology of Nucleic Acids; Inc. was used. Academic: New York, 1976; Vols. I and 11. (10). Proton N M R spectra were recorded on a Varian XL-200 spectrom(2) For a recent review of the role of (6-4) products in ultraviolet light eter using CD,CN as solvent. induced transition mutations, see: Franklin, W. A,; Haseltine, W. A. Mutat. (! 1) 31P{H)spectra were recorded on a Varian XL-200 spectrometer opRes. 1986, 165, 1-7. erating at 80.984 MHz with H,P04 as the standard for chemical shifts. (3) (a) Patrick, M. H . Photochem. Photobiol. 1970, 11, 477-485. (b) (12) Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; PerIkenaga, M.; Patrick, M.H.; Jagger, J. Ibid. 1970, 11, 487-494. (c) Jagger, gamon: New York, 1982, vol. 3, pp 707-708. J.; Takebe, H.; Snow, J. M. Ibid. 1970, 12, 185-196. (d) Ikenaga, M.; Patrick, (13) Cotton, F. A. Inorg. Chem. 1964, 3, 1217. M. H.; Jagger, J. Ibid. 1971, 14, 175-187. (14) Several examples of eight-electron systems containing Mo,(p,-O)(pCI), cores have been reported, namely, [Mo,0CI,(O~CCH3),(H,0)3]ClC10415 (4) Johns, H. E.; Pearson, M. L.; LeBlanc, J. C.; Helleiner, C. W. J. Mol. Biol. 1964, 9, 503-524. and (E~,N),[Mo,OCI~(O~CCH,)~].~~ (5) (a) Nishio, T.; Katoh, A,; Omote, Y.; Kashima, C. Tetrahedron Lett. (IS) Bino, A,; Cotton, F. A.; Dori, Z. Inorg. Chim. Acta 1979, 33, L133. 1978, 1543-1544. (b) Nishio, T.; Kato, A.; Kashima, C.; Omote, Y. J . Chem. (16) Shang, M.; Huang, J.; Liu, J. Acta Crystallogr., Sect. C 1984, C40, 761. Sac., Perkin Trans. I 1980, 607-610.
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